Abundant plant biomass could become a sustainable source of fuels and chemicals. Unlocking this potential requires the economical conversion of recalcitrant lignocellulose into useful intermediates, such as sugars. We report a high-yielding process for the chemical hydrolysis of lignocellulose into monosaccharides. Adding water gradually to a chloride ionic liquid containing catalytic acid leads to a nearly 90% yield of glucose from cellulose and 70-80% yield of sugars from untreated corn stover. Ion-exclusion chromatography allows recovery of the ionic liquid and delivers sugar feedstock that supports the vigorous growth of ethanologenic microbes. Hence, a simple chemical process enables crude biomass to be the sole source of carbon for a scalable biorefinery.
As the primary components of lignocellulosic biomass, the sugar polymers cellulose and hemicellulose are among the most abundant organic compounds on earth and have the potential to be renewable sources for energy and chemicals. The estimated global annual production of biomass is 1×1011 tons, sequestering 2×1021 J [1, 2]. For comparison, annual petroleum production amounts to 2×1020 J, while the technically recoverable endowment of conventional crude oil is 2×1022 J [1]. Hence, in only one decade, Earth's plants can renew in the form of cellulose, hemicellulose, and lignin all of the energy stored as conventional crude oil. The challenge for chemists is to access these polymers and convert them into fuels and chemical building blocks.
Sugars are natural intermediates in the biological and chemical conversion of lignocellulosic biomass [3-8] but access to sugars is hindered by the recalcitrance of plant cell walls [3-9]. The majority of glucose in lignocellulose is locked into highly crystalline cellulose polymers. Hemicellulose—a branched polymer of glucose, xylose, and other sugars—and lignin—a complex aromatic polymer—encase the cellulose, fortifying and protecting the plant. Deriving sugars from this heterogeneous feedstock requires both physical and chemical disruption. Enzymatic methods of saccharification are the most common, and use physical and chemical pretreatment processes10 followed by hydrolysis with cellulases to produce sugars. The proper combination of pretreatment and enzymes for a given feedstock enables high yields of sugars from both hemicellulose and cellulose components11. Nonetheless, the costs of both pretreatment and enzymes (estimated to be as much as one-third of the cost of ethanol production from cellulose, [12]) and low rates of hydrolysis are potential drawbacks to enzymatic hydrolysis.
Exclusively chemical technologies for biomass hydrolysis have also been developed. As early as 1819, Braconnot demonstrated that linen dissolved in concentrated H2SO4, diluted with water, and heated was transformed into a fermentable sugar [13,14]. As in this example, concentrated acid can play a dual role in biomass hydrolysis. By disrupting its network of intra- and interchain hydrogen bonds, strong acids decrystallize cellulose and make it accessible to reagents [15] and by catalyzing the hydrolysis of glycosidic bonds, strong acids cleave cellulose and hemicellulose into sugars (FIG. 1) [3]. Bergius took advantage of these attributes of HCl in the development of a commercial process that operated in Germany from 1935 to 1948 [16, 17]. In the United States, several related processes using H2SO4 have been developed, typically with 80-90% conversion of cellulose and hemicellulose into sugars [18-22]. In a recent example, Cuzens and Farone used concentrated aqueous H2SO4 to hydrolyze agricultural residues via the Arkenol process [23], which is being commercialized by BlueFire Ethanol (Irvine, Calif., USA). In this method, biomass is decrystallized with 77% H2SO4, diluted to a water content of about 40 wt %, and hydrolyzed at 100° C. This first stage hydrolyzes nearly all of the hemicellulose and some of the cellulose. The solid residue is then subjected to a second-stage hydrolysis to release the remaining glucose. Concentrated acid hydrolysis methods produce high sugar yields, use simple catalysts, and require only short reaction times. Despite these advantages, the hazards of handling concentrated acids and the complexities of recycling them have limited the adoption of this technology.
Less hazardous and more tractable cellulose solvents would facilitate lignocellulose hydrolysis. Ionic liquids, salts with melting points near or below ambient temperature, show promise as cellulose solvents for nonwoven fiber production [24] and chemical derivatization [25,26]. Like concentrated acids, ionic liquids comprised of chloride, acetate, and other moderately basic anions disrupt the hydrogen bond network of cellulose and enable its dissolution [25-27]. Recognizing these properties, Zhao and coworkers attempted to hydrolyze cellulose in 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) [28]. Using 11 wt % H2SO4 and 1.75 equiv of water relative to the glucose monomer units of the cellulose (about 1 wt % of the reaction mixture), they obtained a 43% molar yield of glucose after 9 h at 100° C. They also reported a 77% yield of total reducing sugars (TRS) based on a 3,5-dinitrosalicylic acid (DNS) assay, but did not discuss what sugars other than glucose (which is the expected cellulose hydrolysis product) comprised TRS. Zhao and coworkers also report reaction of biomass materials such as corn stover and rice straw under similar conditions, obtaining TRS yields of 66-81% but not reporting glucose yields [28]. Most likely, glucose yields from lignocellulose were no higher than those obtained with purified cellulose.
Several reports from other researchers have followed those of Zhao and coworkers. Schüth and coworkers used solid acid catalysts to depolymerize cellulose in [BMIM]Cl, obtaining mainly water-insoluble oligomers rather than glucose [30]. Recently, Jones and coworkers hydrolyzed pine wood in [BMIM]Cl under low-water conditions, obtaining molar yields of monosaccharides that were typically <20% [31]. Seddon and coworkers studied the reactivity of cellobiose in 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl) and then applied their optimized conditions to pure cellulose and Miscanthus grass, obtaining 50% and 30% glucose yields, respectively [32, 33]. These low glucose yields obtained in ionic liquids reported by Zhao and co-workers contrast with the nearly quantitative yields of glucose attainable from cellulose in concentrated acids and other cellulose solvents [34].
Zhao et al. [53] and published US patent application US 2008/0033187 (published Feb. 7, 2008) report a method for conversion of a carbohydrate in an ionic liquid to produce a furan at a substantial yield. The method involves mixing carbohydrate up to the limit of solubility with the ionic liquid, and heating the carbohydrate in the presence of a catalyst at a reaction temperature and for a reaction time sufficient for conversion to furan at a substantial yield.
U.S. provisional application 61/073,285, filed Jun. 17, 2008, relates to a method for converting carbohydrate or a carbohydrate feedstock to a furan in a polar aprotic solvent in the presence of a halide salt or a mixture thereof and optionally in the presence of an acid catalyst, a metal halide catalyst or an ionic liquid (up to 40 wt %). The carbohydrate feedstock can be lignocellulosic biomass.
Published application US 2009/0062524 (published Mar. 25, 2009) relates to a process for the complete or partial degradation of cellulose by dissolving cellulose in an ionic liquid and “treating it with acid, if appropriate with addition of water.” The amount of acid and water added is adjusted to achieve “complete” or “partial” degradation of cellulose. The application states “the addition of water may be necessary if the water adhering to the cellulose used is insufficient to reach the desired degree of degradation.” Water is added along with acid to the solution of cellulose in the ionic liquid or the ionic liquid acid and water are premixed and the cellulose is dissolved in this mixture. The amount of water to be added is further described:                “the water content of conventional cellulose is in the range from 5 to 10% by weight, based on the total weight of the cellulose used (cellulose+adhering water). By using an excess of water and acid based on the anhydroglucose units of the cellulose, complete degradation as far as glucose is also possible. To reach partial degradation, substoichiometric amounts of water and acid are added or the reaction is stopped at that point.The stoichiometry of the process for complete degradation of cellulose to glucose with respect to water is further discussed “(i)f . . . the cellulose which is on average made up of x anhydroglucose units is to be degraded completely to glucose, then x equivalents of water are required. Here, preference is given to using the stoichiometric amount of water (nanhydroglucose units/nacid=1) (sic, it is believed nwater was intended) or an excess, preferably an excess of >3 mol % based on x.” Certain examples provided in the application state that cellulose was completely degraded, but the glucose yield and the presence or absence of by-products were not reported.        
Published applications WO2009030950 and WO2009030849 (both published Mar. 12, 2009) relate to processes for the preparation of water-soluble cellulose hydrolysis products in which cellulose is mixed with ionic liquids and the resulting solvate or solution is treated with an acid in the presence of water. The acid is reported to have a pKa in water of less than 2 at 25° C. The applications state that                “the hydrolysis reaction requires the presence of one mole equivalent of water for each monomer unit in the cellulose. Cellulose itself contains a certain amount of water, the exact amount depending upon the source and the physical form of the cellulose, usually prepared cellulose contains at least 10-15% by weight water. However, excessively high amounts of water in the reaction mixture may result in either reduced solubility of the cellulose in the ionic liquid, and/or reduced conversion of cellulose to water-soluble hydrolysis products. Preferably the total water content of the reaction system is such that the weight ratio of water to cellulose is from 1:1 to 1:20, preferably from 1:5 to 1:15, especially about 1:10.        
Patent application CN1128981, published Oct. 22, 2008, relates to a process for hydrolyzing cellulose in ionic liquid. In the method, the ionic liquid is said to be used as the solvent, and water, the equivalent weight of which is equal to or more than 1 mol, is said to be used as reactant and inorganic acids, the catalytic amount of which is the stoichiometric amount are said to be used as a catalyst. The reaction is reported to employ normal pressure and temperature between 70 to 100° C. for 2 min to 9 hr. The highest yield reported of reducing sugars was 73% with corresponding yield of glucose of 53%.
Published application WO2009047023 (published Apr. 16, 2009) relates to a process for conversion of cellulose in hydrated molten salts. Molten salts are described as those having a melting point below 200° C. and more specifically refers to hydrates of inorganic salts and hydrates of ZnCl2. The method is said to be applicable to materials containing lignin and hemicellulose in addition to cellulose.
Published application US20090020112 (published Jan. 22, 2009) relates to methods for thermolysis of lignocellulosic materials which comprise combining the lignocellulosic material with ionic liquid and subjecting the mixture to pyrolytic conditions, such as heating to a temperature of about 150° C. to about 300° C., where the heating may be anaerobic, to produce a product which, for example, can be 5-hydroxymethylfurfural, furfural 2-methylfurfural, levulonic acid, levulinic acid or levoglucosensone.
Published applications WO2008112291 and US20080227162 (both published Sep. 18, 2008) relate to a method for dissolving wood, straw and other natural lignocellulosic materials in an ionic liquid under microwave irradiation and/or pressure.
While significant effort has been expended in attempts to improve the yields of desirable products from hydrolysis of lignocelluloses. There remains a significant need in the art for methods which result in high yields of monosaccharides. The present invention provides a high-yielding process for the hydrolysis of cellulose and lignocellulosic biomass which generates easily recovered sugars that are superb feedstocks for microbial growth and biocatalytic ethanol production.